3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) and LTE-Advanced standards are developed to meet growing capacity demands due to rapid expansion of wireless data services. One challenging aspect of these standards is the optimal usage of limited radio resources shared by multiple wireless devices. Specifically, the physical downlink shared channel (PDSCH) is designed to carry downlink data and the physical uplink shared channel (PUSCH) is designed to carry uplink data. In order to determine which Modulation and Coding Scheme (MCS) to use when transmitting to a wireless device using PDSCH, a radio access node estimates a Signal-to-Interference-and-Noise ratio (SINR). A wireless device measures the downlink channel quality and reports this value to the radio access node in terms of a Channel Quality Indicator (CQI) report. A CQI report assumes that if a certain MCS is used, a throughput-optimal target Block Error Rate (BLER), usually defined as 10%, will be achieved in a downlink transmission.
The radio access node translates this CQI to a SINR value. Since there might be some mismatch between the predicted channel quality and the current channel condition, an outer loop adjustment (OLA) is used to adjust the estimated SINR from CQI before being used by the link adaptation:Estimated SINR=SINR from CQI+OLA (1)
The OLA is updated based on the received Hybrid Automatic Repeat Request (HARQ) feedback. If a transmission is successful (receiving an Acknowledgement (ACK)) the OLA will be increased by an up-step value, and if the transmission is not successful (receiving a Negative Acknowledgement (NACK)) the OLA will be decreased by a down-step value. With SINR adjusted by OLA, the BLER of the system will converge to a target value determined by the up-step value and the down-step value:
                              target          ⁢                                          ⁢          BLER                =                              up            ⁢                                                  ⁢            step                                              up              ⁢                                                          ⁢              step                        +                          down              ⁢                                                          ⁢              step                                                          (        2        )            
Equation (2) above defines the fundamental relationship between target BLER, the up-step value and the down-step value. Target BLER is usually a fixed system parameter configured by an operator. For a given BLER target, and assuming that the down-step value is set to a configurable system parameter, the up-step value can be calculated as:
                              up          ⁢                                          ⁢          step                =                  down          ⁢                                          ⁢          step          ×                                    target              ⁢                                                          ⁢              BLER                                      (                              1                -                                  target                  ⁢                                                                          ⁢                  BLER                                            )                                                          (        3        )            
Sometimes a HARQ feedback may contain information for more than one downlink transmission. FIGS. 1A and 1B show two such examples. In Time Division Duplexing (TDD), there are often more downlink subframes than uplink (UL) subframes. Therefore, bundling of HARQ feedbacks is one of two HARQ-ACK feedback schemes defined by 3GPP Technical Specification (TS) 36.213 incorporated herein by reference. With HARQ-ACK feedback bundling, depending on the TDD configuration, several Downlink (DL) HARQ Processes (HPs) are bundled, and only one feedback will be reported on the uplink subframe. This is shown in FIG. 1A where the HARQ feedback for both subframe 0 and subframe 1 are reported in subframe 7. The reported bundled HARQ feedback is ACK if all the HPs in the bundle are ACK, otherwise it will be NACK. For instance, for LTE TDD UL-DL configuration 2 where there are 6 DL subframes, 2 special subframes and 2 UL subframes in a radio frame, there could be a maximum of 4 HPs in the bundle. The number of HPs bundled in a HARQ-ACK feedback depends also on the network traffic, network load, the number of wireless devices in a cell, and the scheduling policy. With bursty traffic (i.e., traffic that is characterized by bursts of high requirements), light traffic load, a larger number of wireless devices in the cell, and proportional, fair scheduling, the likelihood that a wireless device is scheduled in consecutive subframes will decrease. Consequently, the number of HPs bundled in a HARQ-ACK feedback will also decrease.
The bundling of HARQ feedback can also be done in the spatial domain where the HARQ feedbacks for the code words in the same subframe are bundled together. The spatial bundling is performed in HARQ_ACK multiplexing for some configurations of LTE TDD. Also, a wireless device may be configured to receive downlink signals from more than one cell simultaneously, where each cell may use a different carrier frequency to carry a downlink signal. This is called carrier aggregation, in which a cell is also referred to as a carrier. A carrier aggregation capable wireless device may be configured to receive downlink signals from more than one cell but send uplink signals back to one cell only. A cell with which a wireless device initially established a communication and to which the wireless device is sending an uplink signal is called the primary cell (pCell) for the wireless device, while a cell sending an additional downlink signal to the wireless device is called a secondary cell (sCell) for the wireless device. Because of this imbalance between downlink capabilities and uplink capabilities, HARQ feedback may also be bundled in a carrier aggregation mode of operation. This is illustrated in FIG. 1 B, where the wireless device is receiving downlink signals in subframe 0 from a pCell and an sCell but is sending the HARQ feedback for both downlink transmissions in an uplink signal to only the pCell in subframe 4.
With bundled feedback such as HARQ feedback, the target BLER may no longer be throughput optimal. As such, there is a need for outer loop LA with bundled feedback.